The present invention relates to a method and an apparatus for casting ferrous and heavy metal articles in vertically parted sand moulds of a mould-string plant.
GB-848,604 discloses a method and a casting apparatus for casting ferrous products in vertically parted moulds of e.g. green-sand moulds of a mould-string plant like the xe2x80x9cDISAMATICxe2x80x9d moulding plant manufactured and marketed by the applicants. In this known method, the molten metal is supplied to the mould by pouring the molten metal from a ladle into the mould cavity. The filling operation is steered by tilting the ladle, not allowing any control over the flow speed.
In order to meet the demand for ever lighter and cast products, in particular from the automotive industry, there is an increased need for thin-walled cast ferrous products. Due to the inherently relatively thin walls, the liquid metal in the mould cools down rapidly. Therefore the thin-walled products have to be cast with a high flow speed, so that solidification of the liquid metal, before the mould cavity is completely filled, is avoided. The flow speed in the known method is regulated by tilting the ladle to a certain angle. In order to increase the flow speed the angle has to be increased. The resulting high flow speed gives the liquid metal a high inertia, i.e. the kinetic energy which increases to the power of two with the flow speed, which will, particularly with metals which have a high density, cause impact at the moment that the metal comes in contact with the walls of the mould cavity. This impact may, due to said high flow speeds, partly effect local expansions of the mould cavity causing incorrect dimensions and shapes of the resulting casting, and partly cause that the metal penetrates into the interstices between said grains in the mould wall causing the sand grains to xe2x80x9cburn onxe2x80x9d to the surface of the casting; and the high flow speed can cause erosion of the mould.
It is the object of the invention to provide an improved method and apparatus of the kind described with which said problems can be avoided or at least considerably reduced. This object is achieved with the method and the apparatus as described in detail hereinbelow.
With counter-gravity filling, by applying a variable pressure urging the liquid metal upwards into the mould and by varying that pressure, the flow speed can be controlled such that despite a high mean flow velocity the impact is eliminated or at least considerably reduced.
The pressure may be delivered by a pump supplying a gas to a sealed container with molten metal. The pressure urging the molten metal to the mould may, however, also be imparted by a pump, which acts directly on the molten metal.
The metals used with the present invention include iron, steel, copper, lead and alloys thereof.
According to an embodiment of the invention, the flow speed is steered to follow a predetermined value as a function of time.
According to another embodiment of the invention, the reservoir of molten metal is placed below the mould, or at the side of the mould, such that the upper level of molten metal in the reservoir is below the mould cavity.
According to yet another embodiment, the filling process is closed-loop controlled.
According to a further embodiment, the molten metal is delivered by pressurising a sealed container containing the molten metal.
According to another embodiment the moulds are provided with a closure means.